1. Technology of the Invention
The present invention relates to a surface treatment method and surface treatment apparatus and more particularly, to a method and apparatus for selectively etching a silicon oxide film against a silicon nitride film.
2. Background Art
Conventionally, for etching a silicon oxide film, a reactive ion etching method has been employed because it enables a high accuracy pattern formation. In the reactive ion etching method, a substrate to be treated (e.g., a substrate on which a thin film to be etched is formed) is placed in a vacuum container. A pair of parallel plate electrodes is disposed in the container into which a reactive gas is introduced. Electric power of high frequency is applied to the electrodes so that gas discharging takes place to generate gas plasma, which is used for etching the substrate.
In addition to the reactive ion etching method, there are other methods including plasma etching method, ECR dry etching method, ion beam etching method and photo excited etching method. These methods carry out etching by chemically and physically react ions in an activated reactive gas onto the substrate in the vacuum container. Thus, in this respect, these methods are considered to be the same as the reactive ion etching method.
An example of these etching methods will now be described.
In performing selective etching to a silicon oxide film against a silicon nitride film, for example, the reactive ion etching is carried out by electric discharge in a mixture gas of fluorocarbon gas and H.sub.2 or CO gas. Since various sorts of ion species generated in the plasma react with the substrate, the ion species contributing to the etching and the ion species contributing to the deposition co-exist on the substrate, which lowers the efficiency of the etching. In addition, the ion species acting to raise a selectivity of the silicon oxide film to the silicon nitride film have not been confirmed yet and other ion species may be present on the substrate, which deteriorates the selectivity. However, as the integration density of a semiconductor integrated circuit is increased, when performing etching to the silicon oxide film on the silicon nitride film to form contact holes or the like, a higher selectivity against the underlying silicon nitride is required. Suppose now a case where contact holes such as storage node contacts or bit line contacts in a DRAM are formed by using a self aligned contact hole (SAC) etching process as shown in FIG. 20. On the surface and the side walls of a gate electrode 33 of a polycrystalline silicon film which is formed on a silicon substrate 31 via a gate insulating film 32, a silicon nitride film 34 is formed. Above the silicon nitride film 34, a silicon oxide film 35 having a thickness of about 700 .mu.m is formed as an interlayer insulating film. Thereafter, In forming a contact hole H in the gate electrode 33 in a self alignment manner, a resist pattern R having a somewhat larger size is formed and etching is performed by using the silicon nitride film 34 as an etching stop layer. At this time, for the purpose of reducing a capacitance between wiring, the thickness of the interlayer insulating film cannot be made too small and thus must be about 400 nm. The gate electrode is formed in a multi-layered structure with the metallic film and is required to have a thickness of about 300 nm. The thickness of the silicon nitride film 34 is required to be as small as possible since wiring material is embedded into the contact hole after the SAC etching step. In this example, the thickness of the silicon nitride film 34 is about 50 nm.
For the SAC etching, the thickest part of the silicon oxide film in the opening has a thickness of 700 nm. Since the etch rate of the silicon oxide film is unequally distributed on the wafer surface and the thickness of the interlayer insulating film is unequally distributed on the wafer surface, over-etching is necessary by about 30%. In other words, an etching time of about 910 nm (=700 nm .times.1.3) is necessary.
Thus, the time during which the silicon nitride film 34 is subjected to the reactive ion etching (RIE) corresponds to the etching time of the silicon oxide film of 510 nm (=910 nm -400 nm).
At this time, the silicon nitride film must exist at least half of the thickness thereof, which is 25 nm.
Accordingly, the etching selectivity of the silicon oxide film to the silicon nitride film is required to be 20.5 (=510/25).
Thus, by determining the conditions for satisfying the requirements that the etching selectivity of the silicon oxide film to the silicon nitride film is about 20, the cell size can be remarkably reduced. However, it is difficult to cope with such requirements by conventional etching methods. For the purpose of improving the selectivity, variety of studies have been conducted.
In summary, conventional selective etching which use a fluorocarbon gas for selective etching of the silicon oxide film against the silicon nitride film, have a problem that among various sorts of ion species contributing to the surface reaction, it is not confirmed which ion species actually contributes to the surface reaction. As a result, these methods provides low etching efficiency, insufficient selectivity, and therefore impractical.